Current trends in minimally invasive dentistry emphasize the reversal and repair of the active caries process as a first step to restoring the damaged tissue. Enamel remineralization is an accepted phenomenon, and the mechanisms are established. Dentin remineralization appears to be possible, but due to its complex composite structure, it has not been established if proper mechanical function can be restored by remineralization of affected dentin. New methods now permit quantitative studies of the nature of dentin remineralization. We seek to define the structure and property changes associated with dentin remineralization to gain insight into the controlling mechanisms, and aid in defining the "quality" (hydrated elastic modulus and hardness) of the remineralized tissue. The central hypothesis is that bound, intrafibrillar mineral must be present for proper mechanical function. We contend that this will be possible if remnant apatite crystals remain in the intrafibrillar compartment to provide nuclei for the remineralization process within collagen fibrils. In Aim 1. we determine if much slower demineralization occurs for intrafibrillar mineral than for extrafibrillar mineral by cariogenic acids at pH = 5. Aim 2. determines if various zones of dentin caries contain substantial intrafibrillar mineral, and if this is associated with characteristic bulk properties of the intertubular dentin for permanent and primary teeth. Aim 3. evaluates carious teeth successfully treated by ART to determine if they have been functionally remineralized, while Aim 4. evaluates strategies for functionally remineralizing 1) dentin that has been demineralized by cariogenic acids and 2) multiple zones of large natural carious dentin lesions. Clinical treatment of caries (tooth decay) is increasingly more conservative and emphasizes management of risk factors and minimally invasive restoration. Clinicians believe that dentin caries lesions reharden under some circumstances, but are justifiably cautious since the exact conditions are unknown. New approaches emphasizing remineralization are supported by clinical trials showing effectiveness, yet the scientific evidence supporting functional remineralization efforts for dentin caries lesions is weak. We seek to provide the needed foundation that defines the conditions for functional remineralization. This will lead to therapies that will repair the dentin, restore its mechanical properties, and reduce the cycle of surgical-restorative interventions.